Several methods and means for connecting branch conduits via valves to conduits in systems filled with a fluid are previously known. According to one method described in the Swedish patent No. 425 185 the valve is connected to the conduit in such a way that its flow channel will be tangential to the conduit. By means of a completely tight tool fixed to the outlet of the valve a piston could be moved into the flow channel by means of applied forces. The front end of the piston which is shaped as or provided with a separate cutting edge also comprises a portion formed as a hook. During the axial movement of the piston through the flow channel its front end provided with the cutting edge will cut an opening in the conduit. By means of the special design of the piston with a portion formed as a hook the cut-out portion of the conduit will be caught by and maintained in the piston. When the cutting operation has been finished, the piston is brought back to its initial position, the valve is closed and the tool fixed to the outlet of the valve is removed together with the cut-out portion of the conduit. The tool is now replaced by a branch conduit, the valve is opened and the fluid can flow out into the branch.
The design of the front end of the piston with a gap between the cutting edge and a portion formed as a hook for retaining the cut-out portion of the conduit, sets certain requirements on the conduit to which the branching is to be made. The material has to have a certain hardness so that the cut-out portion of the conduit will be fastened in the gap and follow the piston after the cutting operation when the piston is withdrawn into the tool. If the conduit hasn't the assumed hardness but has been subjected to changes due to age or if it has been made red-hot for some reason e.g. while bending, so that its material properties have been changed, there is the risk that the cut-out portion of the conduit will be dropped into the conduit. The cut-out portion can then be transported with the fluid be out of order.